Line similarity in the relative line depth domain for the SDO-based SOAP-GPU simulation using the PHOENIX input seed spectra (see Sect. 2.2). Top left: similarity of line profiles associated with the Doppler component. Bottom left: similar to the top left panel, but displaying only lines with Pearson correlation coefficient values greater than 0.0. Kernel density estimation is used for better visualization of the data distribution. There is a strong linear correlation between the line profile similarity and the relative line depth as the strong lines show higher similarity than the weak lines. This is likely because the strong lines have a higher signal-to-noise ratio than the weak lines. Top right: similarity of line profiles associated with the depth component. Bottom right: similar to the top right panel, but displaying only lines with Pearson correlation coefficient values greater than 0.0. Kernel density estimation is used for better visualization of the data distribution. A strong linear correlation is visible at relative line depth smaller than 0.4, where the lines are photon-limited. Beyond 0.4 the data is more uniformly distributed.